Do you know ghouls from goblins and ghosts?
"pain in the ass," 1811, from medical Latin proct-, Latinized form of Greek comb. form of proktos "anus" (see proctology) + -algia.
proctalgia proc·tal·gi·a (prŏk-tāl'jē-ə, -jə)n. Pain at the anus or in the rectum. Also called proctodynia, rectalgia.